1. Field of the Invention
The present invention relates to an improvement of a gas discharge lamp, which has an internal electrode provided inside a tube bulb having a fluorescent material film formed on its inner wall and a belt-shaped external electrode provided on the outer surface of the bulb in tight contact therewith in the tube axial direction, and applies a high frequency voltage between the internal and external electrodes to cause a discharge inside the bulb.
2. Description of the Related Art
In the above-mentioned gas discharge lamp, discharge material such as a rare gas or mercury or the mixture thereof is filled in the bulb.
The gas discharge lamp as disclosed in U.S. Pat. No. 4,645,979 is known as the prior art of the gas discharge lamp. According to the disclosed gas discharge lamp, one of the electrodes is provided inside the tube bulb as an internal electrode and the other electrode is formed in a belt shape on the outer surface of the bulb in tight contact therewith, as an external electrode. A high frequency voltage is applied between the internal and external electrodes to cause a glow discharge inside the bulb.
The gas discharge lamp may not have a uniform luminance distribution along the tube axis due to the belt shaped external electrode.
In the gas discharge lamp, there occurs a discharge between the various portions of the external electrode and the internal electrode. Accordingly, the longer the discharge distance between the two electrodes is, the lower the current density in the discharging space is and a smaller amount of fluorescent material is excited.
Near the internal electrodes, however, the electrons emitted from the internal electrode are not sufficiently accelerated, so that the ultraviolet ray generated by the discharge is low and the fluorescent material is not sufficiently excited.
As a result, the luminance is high at the center portion of the bulb and is low at both end portions of the bulb.
Since the external electrode itself has a certain impedance, it is likely that the current density becomes higher at that portion of the external electrode that is close to where a lead wire is extracted and becomes lower at a point farther from the location of the lead wire. This would cause the uneven luminance distribution.